Scientists operating Europe's gravitational wave observatories have combined efforts this summer to search for gravitational waves. This groundbreaking research is being taken forward in Europe while similar US-based detectors undergo major upgrade work.

Cataclysmic cosmic events such as supernovae, colliding neutron stars and black holes, as well as more familiar objects such as rotating neutron stars (pulsars) are expected to emit gravitational waves - oscillations in the fabric of space-time predicted by Einstein's General Theory of Relativity. The detection of such waves would revolutionize our understanding of the Universe.

Europe's two ground-based gravitational wave detectors GEO600 (a German/UK collaboration) and Virgo (a collaboration between Italy, France, the Netherlands, Poland and Hungary) have started a joint observation program that will continue over the summer, ending in September 2011.

These detectors work by measuring tiny changes (less than the diameter of a proton), caused by a passing gravitational wave, in the lengths (hundreds or thousands of meters) of two joined arms lying in a horizontal L-shaped configuration. Laser beams are sent down the arms and are reflected from mirrors, suspended under vacuum at the ends of the arms, to a central photodetector. The periodic stretching and shrinking of the arms is then recorded as interference patterns.

"Listening" for gravitational waves benefits enormously from simultaneously deploying two or more such laser interferometers located at different points on the Earth's surface.

In this way, any extraneous, terrestrially generated noise mimicking a genuine gravitational wave signal can be eliminated, since it is unlikely to have the same characteristics at the different locations while the gravitational wave signal would remain the same.

Moreover, just as our brains can work out the direction of a sound source from the difference in signals received by our two ears, detectors in separate locations can help reconstruct the position in the sky of a gravitational wave source. (With two detectors, the most likely sky position lies in a circle; in the case of three or more detectors, it can be pinned down to few spot locations).

"If you compare GEO600 and Virgo, you can see that both detectors have similar sensitivities at high frequencies, at around 600Hz and above", says Dr. Hartmut Grote, a scientist at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) and the Leibniz University in Hannover, Germany.

"That makes it very interesting for us to search this band for possible gravitational waves associated with supernovae or gamma-ray bursts that are observed with conventional telescopes."

Gamma-ray bursts - the most luminous transient events in the Universe - may result from the collapse of a supermassive star core into neutron star or black hole. These phenomena are expected to generate strong gravitational radiation, and so provide ideal references for gravitational wave searches.

The expected frequencies depend on the mass of the objects and may extend up to the kHz band. However, given the weakness of the expected gravitational wave signal, the likelihood of detecting such an event is low. How often such events can be detected therefore depends strongly on the sensitivity of the detectors.

Thanks to its excellent sensitivity at low frequencies (below 100 Hz), Virgo will also search for signals from isolated pulsars such as Vela, the remnant of a massive supernova explosion that emits regular pulses of electromagnetic radiation, from gamma-rays to radio waves. The gravitational wave signal frequency should be at around 22Hz.

In addition, the program will test new technology that will be used in the next (second) generation of gravitational wave observatories.

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